1. Field of the Invention
The present invention relates to a technique of measuring electrical characteristics of semiconductor wafers and more specifically to a technique of carrying out C-V measurement and C-t measurement.
2. Description of the Related Art
The C-t (capacitance-time) measurement is one of the known methods for evaluating semiconductor wafers: for example, the crystal structure of semiconductor wafers. FIGS. 1(A) and 1(B) show a typical process of C-t measurement. The process of C-t measurement first locates a measuring electrode on a semiconductor wafer with an insulating film (or an air gap) between them to construct an MIS (metal-insulator-silicon) structure. As shown in FIG. 1(A), at the time of starting C-t measurement, a voltage Vacc is applied to the measuring electrode to set a target area on the surface of the semiconductor wafer in an accumulating state. This voltage Vacc will be hereinafter referred to as the `accumulation voltage.` An electrostatic capacity measured by the measuring electrode at that time is called `accumulation capacitance Cacc`. A voltage Vmeas is then applied stepwise to the measuring electrode to set the target area in an inverting state. The voltage Vmeas will be hereinafter referred to as the `measurement voltage` or the `inversion voltage`. The C-t characteristic means a transient change of the measured capacitance after the stepwise change of the voltage. The result of C-t measurement is continuously recorded until the measured capacitance reaches an inversion capacitance Cinv of the target area (that is, until a variation in measured capacitance against the time substantially reaches a plateau). A recovery time Tr, which is a time period between the time point when the applied voltage is changed stepwise to the measurement voltage Vmeas and the time point when the measured capacitance reaches the inversion capacitance Cinv, mainly depends upon the crystal structure of the semiconductor wafer and also upon the measurement voltage Vmeas.
A known Zerbst plot shown in FIG. 2 is obtained from the C-t characteristic curve shown in FIG. 1(B). The slope of the plot determines values of a variety of physical parameters, such as a life time of minority carriers in the target area and a surface recombination rate of the minority carriers.
The shape of the C-t characteristic curve obtained by the C-t measurement significantly depends upon the voltages Vacc and Vmeas applied to the measuring electrode. Appropriate levels of these voltages Vacc and Vmeas depend upon the surface condition of the target area (for example, the quantity of movable ions near the surface). More concretely, if movable ions, such as Na.sup.+, are included in an insulating film on the surface of the target area, for example, the flat-band voltage of the target area is shifted by the movable ions, thereby changing the appropriate levels of the voltages Vacc and Vmeas. In the prior art technique, since the C-t measurement is carried out with the voltages Vacc and Vmeas of predetermined constant levels, the measurement sometimes does not give any desired C-t characteristics. When a desired C-t characteristic is not obtained, the operator is required to adjust the voltage levels to carry out the C-t measurement again.